1. Absolutely, get your first character to the end of Chapter 1 (usually around level 30-32). This unlocks your legacy.
2. On my first 50 I did most of the heroics and most of the side missions planetary quests because I wanted to see as much as possible. By the time I got my third alt, I was skipping a lot, and finished my class quest at level 47. However, Belsavis and Ilum dailies give a TON of experience at level 47 -- I was able to do the last 3 levels in one evening that way.
3. I have a Sharpshooter Gunslinger and a Lethality Operative. I like the SS Gunsligner better. When more character slots are available I'll probably roll a Sniper. But, that's just a personal preference and two of my friends would tell you the opposite.
Hope this helps.